Heretofore, a polymer having fluorosulfonyl groups (—SO2F) in a copolymer of a fluorinated monomer of the following formula with tetrafluoroethylene, converted to sulfonic acid groups (—SO3H), has been commonly used as an electrolyte material constituting an ion change membrane (such as a membrane to be used for electrolysis of sodium chloride or for a polymer electrolyte fuel cell) or a catalyst layer for a fuel cell. In the following formula, Y1 represents a fluorine atom or a trifluoromethyl group, n is an integer of from 1 to 12, m is an integer of from 0 to 3 and p is 0 or 1, provided that m+p>0.CF2═CF—(OCF2CFY1)m—Op—(CF2)n—SO2F
In a case where a sulfonic acid group-containing polymer (hereinafter referred to also as sulfonic acid polymer) is used for a fuel cell, it is a polymer capable of improving the power generation energy efficiency, but for further improvement of the performance, a polymer having a lower electric resistance and a high softening temperature is desired for a sulfonic acid polymer. The electric resistance may be lowered by increasing the proportion of the fluorosulfonyl group-containing monomer and increasing the ion exchange capacity of the sulfonic acid polymer. However, with the conventional fluorosulfonyl group-containing monomer, it was difficult to sufficiently increase the molecular weight of a copolymer when it was attempted to increase the proportion of the fluorosulfonyl group-containing monomer used for copolymerization, and there was a problem that the copolymer became swollen too much by water. Therefore, a membrane formed from this kind of copolymer did not have sufficient mechanical strength or durability so that it was not practical for use.
Under the circumstances, to obtain a membrane having a high tetrafluoroethylene content in order to maintain a high ion exchange capacity and secure the strength, it is conceivable to use a monomer having a plurality of sulfonic acid groups or precursor groups capable of being converted to sulfonic acid groups, such as fluorosulfonyl groups, in its molecule.
As such a monomer, the following compound which is a monomer having two fluorosulfonyl groups, is disclosed in Patent Document 1:[FSO2(CF2)a][FSO2(CF2)b]CF-QF-CF2OCF═CF2 
In the formula disclosed in Patent Document 1, a is an integer of from 1 to 3, b is an integer of from 1 to 3, and QF is a single bond or a C1-6 perfluoroalkylene group which may have an etheric oxygen atom. However, it does not disclose a polymer obtained by polymerization of such a monomer or an electrolyte material for fuel cells.
Further, in Patent Document 2, a sulfonic acid polymer obtained by the following monomer is proposed.(XSO2)kCY1(CF2)mO(CFZCF2O)nCF═CF2 
In the formula disclosed in Patent Document 2, k=2 or 3, k+1=3, m=0 to 5, n=0 to 5, X═F, Cl, OH, O(M)1/L (wherein M is a monovalent to trivalent metal, and L is the valence of the metal), OR (wherein R is a C1-5 alkyl group, and such an alkyl group may be one containing an element other than carbon or hydrogen), or A-(SO2Rf)aB (wherein A is nitrogen or carbon, as to a, a=1 when A is nitrogen and a=2 when A is carbon, B is hydrogen or a monovalent metal, and Rf is a perfluorinated alkyl group), Y1=F, Cl or CF3, and Z=F, Cl, CF3, Br or I.
The sulfonic acid polymer has a structure wherein a plurality of sulfonic acid groups are bonded to one carbon atom, and there is concern about stability, as there is no certainty as to whether or not such a structure may have durability over a long period of time.
Patent Document 1: WO2005/003062 (claim 17)
Patent Document 2: WO03/106515 (within the Claim)